Leetcode - Merge Intervals

Merge Intervals

Given a collection of intervals, merge all overlapping intervals.
For example,
Given [1,3],[2,6],[8,10],[15,18],
return [1,6],[8,10],[15,18].

/**
 * Definition for an interval.
 * public class Interval {
 *     int start;
 *     int end;
 *     Interval() { start = 0; end = 0; }
 *     Interval(int s, int e) { start = s; end = e; }
 * }
 */
public class Solution {
    public ArrayList<Interval> merge(ArrayList<Interval> intervals) {
        ArrayList<Interval> res = new ArrayList<Interval>();
        
        if(intervals == null) return intervals;
        int len = intervals.size();
        if(len == 0 || len == 1)    return intervals;
        
        // sort the input by start value
        Collections.sort(intervals, new IntervalComparator());
        
        Interval a = intervals.get(0);
        for(int i = 1; i < len; i++) {
            Interval b = intervals.get(i);
            // if mergeable, merge a and b.
            if(a.end >= b.start) {
                a = new Interval(Math.min(a.start, b.start), Math.max(a.end, b.end));
            }
            // if not mergeable, move to the next, reset the a node.
            else {
                res.add(a);
                a = b;
            }
        }
        res.add(a);
        return res;
        
    }
}

class IntervalComparator implements Comparator<interval>
{
    public int compare(Interval a, Interval b) {
        return a.start - b.start;
    }
}